Historically, the halogen family of elements has been closely linked to the chemical disinfection of water from its very beginnings. Although many alternative chemical disinfectants have been evaluated (e.g. ozone and hydrogen peroxide), chlorine in the elemental or hypochlorite salt forms continues to perform a dominant role in the water treatment field.
The sustained popularity of chlorine in this field for so many years is probably due to two main factors. These are the effective bactericidal action of free available chlorine in water even at relatively low levels and the excellent equipment that has been developed and used for handling chlorine and reliably dosing water with same. In any case, most of the present day potable water treatment plants use some form of chlorine for disinfection and rely upon the maintenance of a very substantial residual free available chlorine concentration in the finished water to insure ultimate delivery of a safe and sanitary product throughout the distribution system. Unfortunately, the inherent energetic reactivity of free available chlorine that is suggested by its strong bactericidal action is also reflected in other ways, notably in its instability and gradual loss from aqueous solutions and its chemical reactivity with a wide variety of both inorganic and organic constituents commonly encountered in raw source waters. As a result, the residual free available chlorine concentration in the finished water released from a modern treatment plant is generally maintained at a level of between about 1.0 and about 3.0 mg./liter, with the particular value chosen being dependent upon the expected rate of disappearance and residence times involved in the distribution system. The levels required tend to vary with water quality, which is subject to weather and seasonal changes due for example to the effects of temperature, sunlight, etc. on reaction rates, solubilities, etc.
In view of these generally accepted practices, the chlorine dosages required for sanitation, expecially in the summertime, frequently reach levels sufficient to create objectionable tastes and odors in the delivered product water. Accordingly and in line with the steady general trend toward minimizing levels of chemical additives in all products intended for human consumption, any measures capabable of reducing the chemical dosages applied in providing safe potable water would be most welcome and beneficial.
Moreover, within the last decade very serious concerns have been raised about the potential health hazards posed by the chlorinated organic compounds identified as trace contaminants in almost all municipal water systems. After due consideration of available scientific data and assessment of probable risks entailed, the U.S. E.P.A. has promulgated regulations setting a maximum level for "total trihalomethanes" (TTHM) in delivered drinking water of 0.10 mg./liter (equiv. to 100 ppb).
Naturally, the research efforts on this problem have consequently been greatly intensified in recent years also, and it has been pretty well established that the trihalomethane contamination problem (principally chloroform) is largely a result of in-situ formation thereof during the water treatment process as a result of the chlorination of organic precursors present in the source water. Some progress in reducing TTHM contamination has been achieved from such research as can be seen from periodical publications like the Journal of the American Water Works Assoc. (J.A.W.W.A.). Representative articles of this type are by Young et al (J.A.W.W.A. 71, 87-95, 1979) showing significant reductions achieved by suitable preclarification of the source water and solids removal prior to the application of chlorine thereto, and by Glaze et al (J.A.W.W.A. 76, 68-75, 1984) showing substantial additional reduction in THM precursors upon supplemental treatment of preclarified water with granular activated carbon and minor further reduction by adding a supplemental ozonization step.
However, such research papers as a whole actually tend to emphasize the extreme difficulty and extra expense involved in trying to meet the prescribed maximum TTHM limit of 0.10 mg./l in the final product water even when starting with a raw source water having only a medium content of organic constituents, i.e. between about 5 and about 10 mg./l of total organic carbon (TOC). Since source waters frequently reach TOC contents as high as about 20 mg./l or more, the urgent need for simple additional techniques and/or other practical and less expensive methods of diminishing the trihalomethane contamination levels in drinking water is readily apparent.